I know that doubles are often used (and sometimes it's really obvious), but I've never heard the term Fake Shemp before.

In the absolutely terrible move Plan 9 from Outer space, Bela Lugosi was listed as a star, but he had died years before the movie was made and the producer used some old clips of him. As Lugosi himself never had any connection with the movie, would he be considered a Fake Shemp?

Shortly after Lugosi's death the story and screenplay for Grave Robbers from Outer Space were written and finalised, with Wood planning to use the unconnected, unrelated footage of Lugosi as a means of putting a credit for him on the picture. Though Wood's actions were driven in part by the desire to give his film a 'star name' and attract horror fans, the Lugosi cameo was also meant as a loving tribute and farewell to the actor, who had become fast friends with Wood in the last three years of Lugosi's life. Wood hired his wife's chiropractor, Tom Mason, as a stand-in for Lugosi, even though Mason was taller than Lugosi and bore no resemblance to him.[2]

Yeah, you got me. There is an actor named Tom Mason, but he's too young to have been a fake Shemp for Lugosi. I don't know why I didn't think of looking for chiropractors named Tom Mason. It seems so logical now.

How can there be multiple Tom Masons? SAG only allows one actor per name. For example, Michael Keaton's real name Is Michael Douglas, which he was not allowed to use because it was already taken. Michael J. Fox had to add a middle initial because there was already a SAG member named Michael Fox (Michael J. Fox's middle name is Andrew, btw).